The present invention relates to ultrasonic imaging devices. More specifically, the invention relates to an ultrasonic imaging device for providing a clear image by sweeping while varying the transmission ultrasonic frequency or by superposing ultrasonic waves of different frequency. The device uses a macromolecular piezoelectric element as a transducer.
Ultrasonic imaging devices are now used in ultrasonic microscopes, ultrasonic diagnosis devices, or ultrasonic flaw detectors, for instance. These ultrasonic imaging devices can be classified into various groups according to the mechanism used. In one of the groups, images are formed by receiving ultrasonic waves reflected by objects. In another group, images are formed by receiving ultrasonic waves transmitted through objects. In still another group, images are formed by receiving both ultrasonic waves reflected by and transmitted through objects. In yet another group, ultrasonic holography is employed in which an ultrasonic hologram is formed by applying a reference wave to an ultrasonic wave reflected by or passed through an object which is then used to form a visible image through an acousto-optic effect.
An ultrasonic wave is greatly attenuated when it passes through a medium. The higher the frequency and the shorter the wavelength, the higher the attenuation. Therefore, an ultrasonic wave of excessively high frequency cannot be used to observe the interior of an object to be examined. For instance, the highest ultrasonic frequency used by an ultrasonic diagnosis device is limited to about ten and several MHz even for examining portions near a surface layer and to about several MHz for examining deeper layers. It is well known in the art that the resolution of the ultrasonic image device is inversely proportional to the wavelength. As the operating frequency is limited as described above, the resolution of these devices is correspondingly limited. In passing through an object to be examined, an ultrasonic wave is diffracted or delayed. Thus, the resultant waves interfere with one another or are irregularly reflected thus creating noise which appears as light and shaded portions or "ghosts". In addition to this, because of factors attributed to the device itself, the actual resolution is lower than the theoretical resolution as determined from the wavelength of the ultrasonic wave. The effective resolution is often several times the wavelength of an ultrasonic wave used.
A method of preventing reduction of resolution caused by noise due to the above-described interference has been described, for instance, in "Acoustical Holography", 5, 373-390 (1974) in an article by Korpel et al. The principle of that method is that, if the wavelength of a generated ultrasonic wave is continuously or stepwise changed so that an image is formed by ultrasonic waves of different wavelength, the noise effects corresponding to the different ultrasonic waves are different from one another. Therefore, only the desired image is emphasized, thereby resulting in a clear image. It is obvious that the principle can be applied to the case where ultrasonic waves of different wavelength are simultaneously generated in superposition. Furthermore, a method in which ultrasonic holograms having various wavelengths obtained by applying a plurality of ultrasonic beams to an object simultaneously or according to a predetermined sequence are made to correspond to light of different hues to form a colored image has been proposed, for instance, in U.S. Pat. No. 3,564,904. Such a colored image can be obtained not only by holography but also with a method in which, in receiving an ultrasonic image with a transducer, received ultrasonic waves of different wavelength are displayed with different colors. Moreover, satisfactory results can be obtained using the method disclosed by Korpel et al or the method disclosed in U.S. Pat. No. 3,564,904 with the ultrasonic frequency range set as large as possible.
A conventional ultrasonic imaging device, in general, employs a non-organic piezoelectric element such as a PZT or a crystal as its ultrasonic transducer. The fundamental resonance frequency f.sub.0 of a piezoelectric element used as an ultrasonic transducer is: EQU f.sub.0 =v/2l, (1)
where l is the thickness of the piezoelectric element and v is the acoustic velocity in the piezoelectric element, in which piezoelectricity of thickness expansion mode is used.
A non-organic piezoelectric element has a conversion efficiency A of several tens of percent in the vicinity of the fundamental resonance frequency f.sub.0. The conversion efficiency A is defined by equation (2). ##EQU1## However, the conversion efficiency A abruptly decreases on either side of the fundamental resonance frequency f.sub.0, that is, the peak conversion efficiency A is obtained at the fundamental resonance frequency f.sub.0.
FIG. 1 shows an example of a measurement which is carried out for determining the variations in conversion efficiency A of a transducer having a piezoelectric element of lead niobate by varying the frequency with the electrical power maintained constant. In FIG. 1, the conversion efficiency A at the fundamental resonance frequency f.sub.0 has a maximum value A.sub.max. As is apparent from FIG. 1, the frequency range in which the conversion efficiency A has values higher than a half of the value A.sub.max is only about 0.6 MHz. Within this frequency range, the conversion efficiency changes considerably abruptly with frequency and therefore received images obtained at the generated frequencies differ in clarity from one another with the result that processing the images is rather difficult.
For a non-organic piezoelectric element of lead niobate or PZT, the impedance and the phase thereof change greatly away from the fundamental resonance frequency f.sub.0 as indicated in FIG. 2. Therefore, if the frequency is varied significantly around f.sub.0, matching a high frequency device driving the transducer to the transducer is of considerable difficulty, and accordingly the necessary means for adjusting or controlling the device is complex. Thus, it is difficult to frequently change the frequency. If the frequency is varied in a range in which at least the phase and the conversion efficiency do not vary much so that the frequency range of the transducer is defined by the maximum conversion efficiency A.sub.max and A.sub.max /2 in that range, then the frequency variation of the lead niobate transducer falls substantially within 3.+-.0.3 MHz.